I'm running YR5's on my BBC. The engine is carbureted, ~9.2:1, headers and moderate hyd roller cam. Q: YR5's appear to be very cold plugs for my application and are routinely very dark in color. I'm checking O2 mixtures shortly with a sensor, but am curious if anyone else out there has had similar findings with this plug range. Q: NGK also lists a UR5 projected nose plug for the same application. Any +/- experience with the UR5 on BBC?